1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for laser Doppler velocity measurements.
2. Background Art
Note that the following discussion refers to a number of publications and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
As is well known, when there is relative motion along the line connecting the source and the receiver of energy propagating as a wave, the frequency of the received energy will be shifted from its original value. This change in frequency due to relative motion is known as the Doppler effect. This effect is readily observed with sound waves, and with electromagnetic waves such as radio waves and light waves.
For laser radars (LADAR) and other laser velocimetry devices, assuming that the receiver is collocated with the transmitter, the frequency of the received signal, fr, is given by the following equation:
                                          f            r                    =                                                    f                0                            ±                              Δ                ⁢                                                                  ⁢                f                                      =                                                            f                  0                                ±                                                      2                    ⁢                                                                                  ⁢                    v                                                        λ                    0                                                              =                                                ∫                  0                                                                                        ⁢                                  (                                      1                    ±                                                                  2                        ⁢                                                                                                  ⁢                        v                                            c                                                        )                                                                    ⁢                                                      (        1        )            where                Δf=the change in laser frequency at the receiver. The (+) applies if the target is moving toward the receiver, and the (−) if the target is moving away.        v=the velocity component along the line of sight transmitter and target.        λ0=the wavelength of the transmitted laser beam.        f0=the frequency of the transmitted laser beam.        c=the velocity of light.        
To determine the velocity component of the target point, it is only necessary to compare fr against f0. A technique for doing this is to combine onto the surface of a photodetector, the received light from the target with the light from a laser local oscillator (LO). The LO beam is generally obtained by diverting a very small fraction of the transmitted laser energy. This process is called heterodyne detection, and the electrical output of the photodetector is the difference frequency between the two laser beams, i.e., Δf. The additional advantage of heterodyne detection is that the amplitude of the received signal is multiplied by the amplitude of the LO, thereby greatly enhancing the signal to noise ratio of the Δf signal.
However, it takes time for light to travel from the transmitter to the target, and return to the receiver. Therefore, fr must be compared with the frequency, f0, that existed when the transmitted wave was first generated, and not necessarily with the f0 that might exist at the time the return signal was received. The difficulty in maintaining a stable local oscillator frequency, even over the very short propagation times of light, lies in the fact that the frequency difference detected is of the order of a few MHz, while the frequency of the light beam is 300 million times greater. This requires extreme mechanical stability of the laser oscillator, even when subjected to environmental effects such as mechanical vibration and thermal distortion.
The methods for solving this problem can be conveniently divided into four categories. For short ranges of a few tens to hundreds of meters, it is relatively easy to build a laser whose frequency drift lies within acceptable limits over the round trip time of the probe beam. For longer distance of involving hundreds of kilometers, it can still be done. However, in this case, the lasers tend to be large and complicated. They are difficult to align and maintain, and there is less freedom in the choice of laser that can be used. U.S. Pat. Nos. 5,796,471, 4,995,720, and 4,589,070 fall into this first category.
The second category involves the equalization of the optical path lengths of the probe beam and the laser LO beam. This technique is often used for applications such as wind tunnel measurements, where the delay can be produced by multiple reflections off of a set of mirrors. For longer ranges, up to several kilometers, a fiber optic coil can be used as the optical path equalizer as describe in U.S. Pat. No. 4,875,770.
A third category utilizes atomic vapor filters. These filters have very steep skirts. The transmission of a laser beam passing through such a filter is a very strong function of wavelength. Very small changes in wavelength produce large changes in the transmission. As a result, a measurement of the normalized transmission through the filter is a direct measurement of the wavelength change. U.S. Pat. Nos. 6,181,412 and 5,502,558 disclose interesting ways for making the measurement.
Finally, a fourth category is one in which the stable reference is something other than a laser. In U.S. Pat. No. 6,388,739, the objective is to measure vibration through the Doppler effect. The stable element, in this case a Mach-Zender interferometer, is part of a feedback element to determine the velocity of a mechanically vibrating object. U.S. Pat. No. 5,838,439 also involves vibration measurements, but the mixing of the return beam takes place within the laser cavity itself, instead of the surface of a photodetector. The advantage of this arrangement is that it is not subject to critical alignment problems. U.S. Pat. No. 5,552,879 utilizes a diffraction grating as the stable reference for determining the Doppler shift.
The present invention improves on the state of the art within this last category.